1. Field Of The Invention
This invention relates to the operation of reformer centrifugal compressors, and more particularly, to improvement in such an operation comprising the injection of nonreactive gas to maintain a substantially constant molecular weight of the recycled gas and thereby increase hydrogen content thereof.
2. Description Of The Prior Art
Catalytic reforming is a process performed to increase the octane number of naphtha. That is, it is done to increase the gasoline yield from crude oil. This is particularly important today since the elimination of lead in gasoline has resulted in a loss in octane. Recycled hydrogen is fed into a naphtha feed by a compressor, and the mixed feed is charged to a reforming reactor or a series of such reactors in which the mixture of hydrogen and charge vapors is contacted with a reforming catalyst. Either fixed bed or fluidized bed systems may be used. The effluent from the reforming reaction is usually cooled and partly condensed in a flash or separator drum. Condensed fluids or reformate are withdrawn from the drum and further processed as desired. Hydrogen-rich tail gas from the separator drum may be used for hydrogen services or fed back to the recycle compressor.
It is desirable to maximize the hydrogen flow through the compressor, because the reforming catalyst life is very dependent on the hydrogen-to-hydrocarbon recycle ratio. The normal metal catalysts lose activity in the presence of sulphur and nitrogen compounds and are poisoned by metals such as arsenic and lead. Hydrogen fed into the feed, or hydrotreating, removes the nitrogen, sulphur, oxygen and metals, and this helps protect and improve the performance of the catalyst. The advantage of the reforming process is that large quantities of net hydrogen are produced which greatly facilitates the hydrotreating. In addition, hydrotreating of the feed also improves the yield and quality of the reformate and increases the time between regenerations.
The compressors normally used are of the centrifugal type. A problem is that the recycle rate drops as the molecular weight of the recycled gas drops. In other words, by increasing the hydrogen content, the recycle rate will drop because the hydrogen reduces the molecular weight of the recycled gas. To compensate with a centrifugal compressor is a problem because, while the capacity is directly proportional to speed, the head is proportional to the square of the speed, and the brake horsepower is proportional to the cube of the speed, as is well known. Further, the compressor has an overall head limitation, so by increasing speed to increase capacity, the head limitation is reached much more quickly than the desired capacity. Larger compressors with larger drivers could be used, of course, but this may not be cost effective.
One attempt to solve this, problem is disclosed in U.S. Pat. No. 2,849,379 to Hengstebeck. In this patent, control of the recycle compressor is exercised by maintaining compressor speed substantially constant at a constant discharge pressure by controlling the temperature in the flash zone so as to maintain the molecular weight of the recycle gas substantially constant while maintaining the pressure drop of the gas through a flow control orifice constant. The flow of recycled gas from the compressor is controlled by a flow controller which may take the form of a conventional orifice meter and a controller which controls the compressor speed. A second controller monitors compressor speed and controls the flow of cooling water through a cooler through which the gas is fed to the flash drum.
The present invention increases the hydrogen recycle and catalyst life by the injection of non-reactive gas into the hydrogen recycled gas fed to the compressor. The complicated control system of the Hengstebeck patent is not needed.